Field of the Invention
The invention relates to an extruder die head, preferably a blown film head, comprising an internal cylindrical mandrel and a jacket, which envelops concentrically the mandrel. Between the mandrel and jacket an annular channel is formed that empties into a die slit. The invention also comprises at least one line, which empties into the annular channel in the area opposite the die slit and which feeds a melt.
Blown film heads of this type exist. Usually the mandrel exhibits one or several helical groove(s), whose depth decreases in the direction of the die slit, starting from the feed channel(s), so that the polymer melt overflows more and more the webs, which define the channels, and assumes a uniform flow in the axial direction. The prior art extruder die heads exhibit the special problem that the mandrel on the other side of the beginning of the annular channel is fitted sealingly into a cylindrical borehole of the jacket. If, however, the extruder die head becomes warm due to the polymer melt, flowing through the extruder die head, the jacket expands more, on account of its larger diameter, than the central mandrel so that between the two of them a slit forms, into which penetrates the polymer melt that is fed in under pressure. Since the melt can accumulate in the area of this slit and because of its long residence time in the hot die head, where it can decompose and become brittle, particles of the deposited and incrusted melt can be dragged along. The particles become obvious in the extruded film tube or the inflated film bubble as the defective spots.
Therefore, the object of the invention is to provide an extruder die head of the type designated in the introductory part, wherein there is no slit between the central mandrel and the jacket, enveloping said mandrel, in which the polymer melt can accumulate and deposit.
The invention solves this problem by means of a first embodiment in that the mandrel is made as one piece with a flange-shaped foot and the jacket rests with its bottom face sealingly on the annular surface of the flange-shaped foot and is connected, for example fastened, to the same; that the annular channel extends from the peripheral surface of the mandrel to the flange-shaped foot up to and into the transition region; and that the line, feeding in a melt, empties into this transition region.
Since the bottom face of the jacket rests sealingly on the annular surface of the flange-shaped foot and since the mating surfaces of complementary shape are connected permanently together and preferably braced, there is no slit between the face and the annular surface, into which the melt could penetrate.
A preferred embodiment provides that the flange-shaped foot, adjacent to the mandrel, is provided with one or several spiral grooves, which run helically over the transition region into the peripheral surface of the mandrel and whose depth decreases in the direction of the die slit and which have their start in the foot and in whose initial region the boreholes, feeding in the melt, empty. In this embodiment the edges of the groove(s), worked into the flange-shaped foot, are sealed by means of the faces, which cover said grooves and belong to the jacket, opposite the annular surface of the foot.
A second embodiment solves the problem in that the mandrel is put into a blind borehole of a die body, which forms the jacket and whose inside wall defines with the mandrel the annular channel; that the channel extends between the peripheral surface of the mandrel and its face up to and into the transition region; that the melt-feeding line empties into this transition region; and that the face of the mandrel inside the transition region rests sealingly on the floor of the blind borehole and is connected, for example fastened, to the same.
Preferably the peripheral region of the face of the mandrel is provided with one or several spiral grooves, which run helically over the transition region into the peripheral surfaces of the mandrel and have their start in the peripheral region of the face and whose depth decreases in the direction of the die slit; and that the melt-feeding boreholes empty into the initial regions of the grooves.
A third embodiment solves the problem with an extruder die head, preferably a blown film head, which comprises a die body, which forms the floor and on which an inside rotationally symmetrical core with sealing face is mounted and connected, for example fastened, to the same, and comprising at least two rings, which envelop the core and whose faces rest sealingly against complementary annular surfaces of the floor and are connected, for example, fastened, to the same, whereby the core with the ring, enclosing it, and the rings, which enclose each other, define annular channels, which empty into a common die slit, whereby the annular channels extend up to and into the transition regions of the peripheral surface of the core and the rings to their faces and whereby the melt-feeding channels empty into these transition regions.
This extruder die head of the invention enables a coextrusion process, whereby between the faces of the core and the rings, enclosing said core, and the floor there are no slits, into which the melt could penetrate.
A preferred embodiment provides that the peripheral regions of the faces of the core and the rings are provided with one or several spiral grooves, which run helically or spirally over the transition regions into the peripheral regions of the mandrel or the rings and have their start in the peripheral regions and whose depth decreases in the direction of the die slit, whereby the melt-feeding boreholes empty into the initial regions of the grooves.
Expediently the transition regions are rounded off so that the melt can flow with laminar flow over them and beyond.
Expediently the faces are fastened by means of expansion screws to the floor or annular surfaces.
The faces, which mate sealingly, are complementary to each other and designed preferably flat.
If the extruder die heads are blown film heads, there are additionally boreholes, (not illustrated) which penetrate the blown film head and through which blowing air can be fed and also exhausted again for the purpose of inflating the extruded film tube into a tubular bubble and for the purpose of cooling.
These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.